1. Field of the Invention
The invention relates generally to electrical apparatus, and more particularly, to circuit interrupters having interchangeable trip units.
2. Description of the Prior Art
Circuit breakers are widely used to protect electrical circuits and apparatus from damage due to overcurrent conditions. Early circuit breakers usually provided trip units having thermal and magnetic mechanisms to trip the breaker upon overcurrent conditions. Some of these breakers provided interchangeable trip units to allow the trip current rating to be changed without substituting an entire circuit breaker.
As electrical distribution circuits became more complex, it was necessary to provide coordination and interaction between circuit breakers so that breakers on various portions of the system would trip at different levels and with different time delays following occurrence of a fault. This flexibility was provided by shunt trip mechanisms using electronic circuits to analyze a signal produced by a current transformer or other sensing means to generate a trip command to the shunt trip mechanism upon occurrence of a predetermined overload current. An example of such a circuit breaker employing a trip unit with an electronic circuit is described in U.S. Pat. No. 3,826,951 issued July 30, 1974 to A. E. Maier and A. B. Shimp and assigned to the assignee of the present invention.
Circuit breaker parameters which can be varied to suit the application include the level of fault current which can be withstood without physical damage to the circuit breaker frame, the provision for ground fault detection, and the maximum continuous current which the circuit interrupter can withstand without suffering damage. In order to reduce the total cost of manufacturing and supplying a circuit interrupter, it is desirable to minimize both the tooling required in manufacturing the device and the number of types of devices needed to serve in a wide variety of applications. Circuit interrupters having electronic trip units lend themselves to economies in this manner by the ability to provide a variety of interchangeable circuit configurations having common mechanical specifications. However, this raises the possibility of installing a trip unit having circuitry which is not compatible with the electrical configurations of the circuit interrupter. It is desirable to provide a circuit interrupter having removable interchangeable trip units and the capability to prevent an improper trip unit from being installed in the breaker. It is further desirable to ensure that the contacts of the circuit interrupter will remain in an open position unless a proper trip unit is completely installed in the circuit breaker.